In a clean glass jar or bowl, combine the heavy cream and whole milk. Stir gently to blend.

Add the vinegar or lemon juice to the cream mixture and stir gently just until combined. Don't overmix.

Cover the container with a clean kitchen towel or coffee filter secured with a rubber band. This allows the mixture to breathe while keeping dust and insects out.

Place in a warm spot in your kitchen (around 70-75°F is ideal) and let it stand undisturbed for 24 hours. During this time, beneficial bacteria will multiply and thicken the cream.

After 24 hours, check your sour cream. It should be noticeably thicker with a tangy aroma. If desired, stir in a pinch of salt to enhance the flavor.

Transfer to an airtight container and refrigerate. The sour cream will continue to thicken slightly in the refrigerator.